Abstract: An apparatus for growing thin films onto a substrate by subjecting the substrate to alternately repeated surface reactions of vapor-phase reactants for the purpose of forming a solid-state thin film. The apparatus includes a reaction chamber pack into which the substrate is placed. The apparatus further includes at least two reactant sources, from which the reactants can be fed in the form of gas-phase pulses into the reaction chamber pack. The apparatus further includes reactant inflow channels for connecting the reactant sources to the reaction chamber pack. The reaction chamber pack, the reactant sources and the reactant inflow channels are all placed inside the same pressure shell. The reaction chamber pack and the reactant sources are each provided with individual heaters, which heat the reaction chamber pack and each reactant source, so that each heater is independently controllable from each other. Further, active thermal insulation elements thermally isolate the heaters from each other.